The invention relates to the drive device art and more specifically to drive devices having a drive to be activated by the supply of energy and which deliver drive power.
Such a drive device is for example disclosed in the German utility model 29,903,825.4, where it is described as a component of a toggle clamping device. It comprises a pneumatic drive able to be operated by compressed air with its associated electrically actuated control valves in order to set the direction of driving of the pneumatic drive. As an alternative a hydraulic drive would be possible as well which is connected with electrically actuated servo valves in order to influence the state of the drive. While in the case of pneumatic drives design must be generally technically complex owing to the compressibility of the operating medium if accuracy of positioning and slow motion are to be possible, with hydraulic drives the principal problem is that of leakage and the large amount of upkeep work needed to ensure reliable hose connections and maintaining a high quality hydraulic medium in the system.
In the holding device sector designs with an electrical drive are therefore utilized as an alternative as well, a so-called xe2x80x9celectrical clampxe2x80x9d being described by the company Txc3xcnkers Maschinenbau GmbH, whose electrical drive is in the form of a lead screw drive. However there is still a substantial wear problem here, more particularly when transmitting heavy setting forces.
One object of the invention is to create a drive device with which high drive forces may be transmitted while reducing the rate of wear and the need for servicing.
In order to achieve these and/or other objects appearing from the present specification, claims and drawings, in the present invention a drive device comprises a closed hydraulic circuit which includes a hydraulic drive able to be actuated by a hydraulic medium and a hydraulic pump causing supply and removal of the hydraulic medium to and from the hydraulic drive, an electric motor being provided for operation of the hydraulic pump and the actuation of the hydraulic drive is set by the operational state of the hydraulic pump.
It is in this manner that an electro-hydraulic drive device is created, in which owing to a closed hydraulic circuit the leakage problem may be extremely readily gotten under control and the special control by the electric motor activated hydraulic pump means that no expensive servo valves are required to operate the hydraulic drive in the desired manner. Dispensing with servo valves does in this respect offer the advantage as well that there are only relatively modest requirements for servicing of the hydraulic medium, this meaning that servicing of the equipment is extremely economic. The activation of the hydraulic drive is preferably only made dependent on the operational state of the hydraulic pump and may for example be controlled in a vary simple manner for instance by switching on and off and presetting a certain speed of rotation of the pump.
Further advantageous developments of the invention are defined in the claims. Different actuating pressures required during operation of the hydraulic drive may be conveniently preset in a manner dependent on the speed of rotation of the hydraulic pump. Thus loads can be accelerated or retarded without having to have recourse to an intermediately placed servo valve means, which influences the flow cross section. In this respect use is preferably made of suitable setting means, which may be controlled or regulated by way of a variable preset of the speed of rotation of the electric motor determining the hydraulic pump""s speed of rotation. A possibility may also be provided for setting speed of rotation change functions in order to fashion the acceleration and retardation of a load to the moved by the hydraulic drive in a smooth manner and to avoid jerky motion.
In keeping with a particularly preferred form of the drive device the hydraulic drive is provided with at least one drive piston coupled in a driving manner with a power or force output part, which divides two working chambers from one another in a fluid-tight manner, which are respectively connected by way of a hydraulic circuit with the hydraulic pump, the supply of hydraulic fluid into the respective working chamber being accompanied by the simultaneous flow of hydraulic fluid from the other working chamber in order to displace the drive piston in the desired manner. Since the hydraulic pump is able to be rotated clockwise or counter-clockwise, for instance by changing the direction of rotation of the electric motor or by the use of an intermediate transmission, it is possible for hydraulic medium to be supplied into the one or the other of the two working chambers in order to influence the direction of motion of the drive piston accordingly.
The two hydraulic circuits of the drive device preferably contain a respective overridable check valve, which normally permits fluid flow from the hydraulic pump to the hydraulic drive and prevents it in the opposite direction, each check valve being able to be overridden by the pressure maintained in the respectively other hydraulic circuit by the hydraulic pump in order to render possible fluid flow from the hydraulic drive back to the hydraulic pump. It is in this manner that any desired intermediate positions of the drive piston may be maintained without the constant supply of energy, because the hydraulic medium is trapped by the check valves in the working chambers when the hydraulic pump is not activated. If on the contrary the hydraulic pump is activated, the pressure then established in the one hydraulic circuit overrides the check valve, located in the other hydraulic circuit and accordingly renders possible free movement of the working piston.
A further particularly advantageous design of the drive device is one in which at least one and preferably both hydraulic circuits contain a biasing valve, which normally shuts off the fluid connection from the associated working chamber to the hydraulic pump and only opens it, when and as long as a predetermined opening pressure is established. Thus the biasing valve is responsible for biasing of the hydraulic medium located in the output working chamber, which medium can not be immediately displaced, when there is an increase in pressure in the input chamber. It is only when the increase in pressure in the input working chamber is so strong that the pressure building up in the output working chamber reaches the minimum pressure, termed the opening pressure, that the previously entering hydraulic medium may leave. Since the pressure obtaining in the output working chamber then however produces a constant opposing opposite force to the desired direction of movement of the drive piston, the drive piston may be extremely quickly and accurately retarded even in the case of a very dynamic movement simply by varying the operational state of the hydraulic pump to change the pressure applied on the input side. Therefore even without servo controlled hydraulic valves extremely exact positioning of the drive piston or, respectively, of a force or power output connection member coupled therewith can be achieved even at high speeds of operation.
The design of the pilot valves is preferably such that the opening pressure responsible for opening is between 10% and 90% of the maximum possible operational pressure produced by the hydraulic pump. The preferred pressure range is in this respect between 30% and 50% of the above mentioned maximum actuating pressure. In a manner different than a simple check valve, which opens even at extremely low pressure differentials, the biasing valves are responsible for substantial biasing effect. In this case the opening pressure may be conveniently predetermined with a certain range of variation by suitable adjusting means in order to be able to perform simple adjustment to suit a specific case of application.
It is convenient for the respective biasing valve to comprise a moving shut off valve member, which is biased by spring force corresponding to the desired opening pressure into a closed position interrupting the fluid path and which is acted upon by the hydraulic fluid of the output working chamber opposing the spring force in the opening direction. If the pressure in the output working chamber increases to at least the opening pressure, there will be a resulting opening force able to overcome the spring force and switch over valve member into an open position thereof. The biasing valve consequently preferably possesses an inherent digital switching characteristic or behavior.
If a hydraulic circuit possesses both an overridable check valve and also a biasing valve, such valves will be preferably connected in series, the biasing valve preferably being located between the overridable check valve and the hydraulic drive.
Each biasing valve is preferably placed in parallel with a check valve adapted to open in the direction toward the hydraulic drive and to close in the opposite direction, the check valve rendering possible supply of the hydraulic medium into the associated working chamber, given the right direction of rotation of the hydraulic pump, bypassing the biasing valve.
For compensation of temperature variations and/or different volumes of the working chambers each hydraulic circuit may be connected with a hydraulic fluid equalizing container, which possesses a moving wall subject to the pressure of the atmosphere.
It is convenient for at least the hydraulic drive, the hydraulic pump, the hydraulic circuits and the electric motor to be arranged together as an assembly (drive unit) it being possible to exclusively use electrical interface means for power supply, such interface means serving for the operation of the electrical motor. It is possible to do without hydraulic interface means, because the closed hydraulic circuit may be designed in the form of a self-contained component of the drive unit.
In the case of a particularly advantageous design the drive device is designed in the form of a component of a clamping device, more especially a toggle clamping device, in which the force output part of the hydraulic drive is drivingly connected with a pivoting clamping arm of the clamping device. This design is to be more particularly recommended in conjunction with a drive device in the form of a single drive unit, since this form makes extremely compact dimensions and furthermore use as an alternative to a purely fluid power or purely electrically operated clamping device possible.
Further advantageous developments and convenient forms of the invention will be understood from the following detailed descriptive disclosure of one embodiment thereof in conjunction with the accompanying drawings.